Wafer polishing apparatus

ABSTRACT

The present invention is a wafer polishing apparatus with a device that detects the polishing end according to a change in rotation resistance of a wafer during polishing. The wafer polishing apparatus comprises a head body and a carrier which are connected with each other by a connecting bar which is provided with a strain gage, and determines strain in a horizontal direction of the connecting bar by the strain gage so as to determine a rotation torque of the carrier, whereby accurately detects the polishing end. Since a retainer ring is mounted to the outer periphery of the carrier via an O ring, the impact at contacting of the wafer with the retainer ring can be absorbed by the O ring, thus the wafer is prevented from damage. Because the retainer ring is mounted to the carrier without a gap, the outer periphery of the wafer is enclosed by the retainer ring, and the wafer can be polished in a state where its center is held on the central axis of the carrier. Therefore, the polishing precision of the wafer can be improved.

This application is a divisional application of prior application Ser.No. 09/433,310, filed Nov. 3, 1999, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a wafer polishing apparatus,and more particularly to a wafer polishing apparatus with a chemicalmechanical polishing method (CMP).

2. Description of Related Art

A wafer polishing apparatus with a CMP polishes an oxidized membrane anda metal membrane, and so forth, which are formed on a wafer, and such apolishing apparatus requires a precise control of a polishing amount.Thus, as an apparatus for controlling the polishing amount, that is, asan apparatus for detecting an end of the polishing, an apparatus issuggested which determines a rotation torque of a wafer holding head andthen detects the end of the polishing according to the rotation torque.

As shown in FIG. 4, the apparatus for detecting the end of the polishingdetects, focussing on a difference in rotation resistance (processingresistance) which occurs due to different processing resistances atlayers and different types of polishing liquids (slurries), the changein the processing resistance and finishes processing at an intendedlayer. The relationship between the each layer and the rotationresistance is “metal layer”<“membrane between layers”<“barrier metallayer”.

However, because the apparatus for determining the rotation torque ofthe wafer holding head according to a change in electrical resistance ofa motor determines the rotation torque including the rotation resistanceof a deceleration mechanism connecting an output shaft of the motor andthe wafer holding head, the rotation torque of only the wafer holdinghead cannot be determined exclusively; thus the end of the polishingcannot be detected accurately.

A wafer polishing apparatus disclosed in Japanese Patent ApplicationLaid-open No. 9-19863 forms a pressurized air layer between a carrierand a wafer, and presses the wafer against a platen through thepressurized air layer to polish the wafer. Moreover, according to thewafer polishing apparatus, a retainer ring is arranged at an outerperiphery of the carrier; thus, the retainer ring prevents the waferfrom jumping out of the carrier during polishing. In other words, theouter periphery of the wafer during polishing is contacted with an innerperiphery of the retainer ring whereby preventing the wafer from jumpingout of the carrier.

Directed to a case in which the retainer ring is directly fixed to theouter periphery of the carrier, an impact at contact of the wafer to theretainer ring is not absorbed to the retainer ring side, and such animpact rather becomes a counterforce applied to the wafer; as a result awafer may be damaged.

In view of the above-described adverse effect, the wafer polishingapparatus has the retainer ring arranged with a gap with respect to theouter periphery of the carrier, so as to prevent the wafer from a damageby absorbing the impact with the gap.

Further, the wafer polishing apparatus has the retainer ring arrangedwith the gap with respect to the carrier; thus the wafer is polished,its position with respect to the carrier always moving.

In the wafer polishing apparatus, the wafer is polished preferably in astate that the center of the wafer is held on the central axis of thecarrier in purpose of improving the polishing precision of the wafer.

However, the conventional wafer polishing apparatus has a difficulty inimproving its polishing precision of the wafer since the position of thewafer always changes.

This problem can be solved by fixing the retainer ring directly to theouter periphery of the carrier and polishing the wafer without the gap,but the wafer may be damaged without the gap as described above.

SUMMARY OF THE INVENTION

The present invention has been developed in view of the above-describedcircumstances, and has as its object the provision of a wafer polishingapparatus with a detecting device which accurately detects the end ofthe polishing according to the change in a rotation resistance of thewafer during polishing.

Further object of the present invention is the provision of the waferpolishing apparatus, which can prevent damage of the wafer which occursby the impact at contact of the wafer with a retainer ring, whileimproving the polishing precision by polishing the wafer in the statethat the center of the wafer is held on the central axis of the carrier.

To achieve the above-mentioned object, the present invention is directedto a wafer polishing apparatus, comprising: a platen which rotates; aholding head which holds a wafer and presses the wafer against theplaten to polish the wafer, the holding head comprising a head bodywhich is rotated and arranged to face the platen, a carrier which isarranged below the head body and holds the wafer at its bottom face, aconnecting member which connects the head body with the carrier andtransmits a rotation force from the head body to the carrier, and atorque determining device which is provided to the connecting member anddetermines a rotation torque of the carrier applied from the carrier tothe connecting member; and a polishing amount determining device whichdetermines a polishing amount of the wafer according to the rotationtorque determined by the torque determining device.

Moreover, to achieve the above-mentioned object, the present inventionis directed to a wafer polishing apparatus, comprising: a platen whichrotates; a holding head which holds a wafer and presses the waferagainst the platen to polish the wafer, the holding head comprising ahead body which is rotated and arranged to face the platen, a carrierwhich is arranged below the head body and holds the wafer at its bottomface, a connecting member which connects the head body with the carrierand transmits a rotation force from the head body to the carrier, and afriction force determining device which is provided to the connectingmember and determines a friction force in a rotation direction of theplaten which occurs at the wafer and the platen and is applied from thecarrier to the connecting member; and a controller which controls waferpolishing according to a change in the friction force determined by thefriction force determining device.

According to the present invention, the head body and the carrierconstituting the holding head are connected with each other by theconnecting member which comprises a torque determining device whichdetermines only a rotation torque of the carrier; thus a polishing endcan be accurately detected.

Further, according to the present invention, a strain measuring devicefor measuring the strain of the connecting member is applied as thetorque determining device. The processor calculates the rotation torqueof the carrier according to the strain measured by the strain measuringdevice. Therefore, the accurate rotation torque of the carrier can beobtained.

According to the present invention, a strain gage is applied as thestrain measuring device of the torque determining device, so that therotation torque of the carrier can be easily determined. In that case,the processor calculates the rotation torque of the carrier according toan electric signal outputted from the strain gage.

Still according to the present invention, the head body and the carrierof the holding head are connected with each other by the connectingmember which is provided with a friction force determining device. Thefriction force determining device determines the friction force in arotation direction of the platen, which occurs by the wafer and theplaten, and the controller controls wafer polishing according to thefriction force.

In wafer polishing by CMP, a friction coefficient of the wafer and theplaten changes when a dressing condition of the platen is poor and apolishing rate drops. The friction coefficient tends to be low when thepolishing rate is high, and tends to be high when the polishing ratedrops. Therefore, the present invention for detecting the change in thefriction coefficient, that is, change in the friction force, storesbeforehand the friction force indicating when the platen needs to bedressed, and the friction force indicating the platen needs to bereplaced; thereby the polishing apparatus can automatically determinethe time for dressing and replacement of the platen.

The present invention applies parallel springs as the connecting member,and applies as the friction force determining device for determining thedisplacement between the parallel springs. The friction force caused bythe wafer and the platen is calculated by the processor according to thedisplacement of the parallel springs; thus, the friction force can beobtained easily with a simple structure.

The present invention applies a member to deform elastically by thefriction force, and applies as the friction force determining device fordetermining an amount of elastic deformation of the member. The frictionforce caused by the wafer and the platen is calculated by the processoraccording to the amount of elastic deformation of the member; thus thefriction force can be obtained easily with a simple structure.

The present invention applies a differential transformer as thedetermining device so that the displacement between the parallel springscan be easily determined, and thus the friction force can be easilyobtained.

According to the present invention, the connecting member is arranged onthe central axis of the head body and the carrier, so that the frictionforce in the rotational direction of the rotation torque of the carrier,or the platen can be received on the central axis of the holding head.Therefore, the torque detecting mechanism in a smaller size is possiblethan the one in a case to transmit the rotation torque with the outerperiphery of the holding head. In addition, because the pressing forceand the rotation resistance can be separated, the friction force can beaccurately determined.

Further, to achieve the above-mentioned object, the present invention isdirected to a wafer polishing apparatus, comprising: a platen whichrotates; and a holding head which holds a wafer and presses the waferagainst the platen to polish an obverse of the wafer, the holding headcomprising a head body which is rotated and arranged to face the platen,a carrier which is supported to the head body to be movable vertically,an air blowing member which is provided at a bottom face of the carrierand blows air toward a reverse of the wafer to form a pressurized airlayer between the carrier and the wafer, a pressing device which pressesthe carrier toward the platen to press the wafer against the platenthrough the pressurized air layer, and a retainer ring which is mountedon an outer periphery of the carrier via a buffer member and pressedagainst the platen during polishing the wafer to prevent the wafer fromjumping out of the carrier while enclosing the wafer to hold a center ofthe wafer on a central axis of the carrier.

According to the present invention, the retainer ring is mounted to theouter periphery of the carrier via the buffer member, so that the impactat contact of the wafer with the retainer ring is absorbed by the buffermember. According to the present invention, the retainer ring is mountedto the carrier without a gap; thus the wafer can be polished in a statewhere the outer periphery of the wafer is enclosed by the retainer ringand the center of the wafer is held on he central axis of the carrier.Therefore, polishing precision can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature of this invention, as well as other objects and advantagesthereof, will be explained in the following with reference to theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures and wherein:

FIG. 1 is a view showing the entire structure of a wafer polishingapparatus in the first embodiment;

FIG. 2 is a longitudinal section view of a wafer holding head of thewafer polishing apparatus in FIG. 1;

FIG. 3 is a plan view showing a position to attach a connecting bar withrespect to a carrier;

FIG. 4 is a graph indicating the relationship between a change inprocessing layer and a processing resistance of the wafer with respectto polishing time when polishing the wafer;

FIG. 5 is a longitudinal section view of a wafer holding head in thesecond embodiment;

FIG. 6 is an explanatory view of a movement of a parallel spring whichis attached to a wafer holding head in FIG. 5;

FIG. 7 is a view showing detection signals outputted from a differentialtransformer;

FIG. 8 is a longitudinal view of a wafer holding head in the thirdembodiment; and

FIG. 9 is an enlarged view of a pin and a differential transformerattached to the wafer holding head in FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A description will hereunder be given of an embodiment of a polishingapparatus according to the present invention with the accompanyingdrawings.

FIG. 1 is a view showing the entire structure of a wafer polishingapparatus 10 in the first embodiment.

The wafer polishing apparatus 10 in FIG. 1 comprises a platen 12 and awafer holding head 14. The platen 12 is disk-shaped, and a polishing pad16 is provided thereon. The lower portion of the platen 12 is alsoconnected to a shaft 18 which is connected to an output shaft (notshown) of a motor 20. Thus, when driving the motor 20, the platen 12 isrotated in direction A in FIG. 1, and slurry is supplied on thepolishing pad 16 of the rotating platen 12 from a nozzle (not shown).

FIG. 2 is a longitudinal section view of the wafer holding head 14. Thewafer holding head 14 shown in FIG. 2 comprises a head body 22, acarrier 24, a guide ring 26, a retainer ring 28, a rubber sheet 30, andso forth.

The head body 22 is disk-shaped, and on its top a rotation axis 32 isfixed to be coaxial with its central axis. The rotation axis 32 isconnected to an output shaft of a motor (not shown), and the drivingforce of the motor rotates the head body 22 in direction B in FIG. 2.

Air supply passages 34 and 36 are formed in the head body 22. As shownby an alternative two long and short dashes lines in FIG. 2, the airsupply passage 34 is extended outside the wafer holding head 14, and isconnected with an air pump 40 via a regulator 38. Similarly, the airsupply passage 36 is also extended outside the wafer holding head 14,and is connected with the air pump 40 via a regulator 42.

The disk-shaped carrier 24 is arranged at the bottom of the head body 22to be coaxial with the central axis of the head body 22. Moreover aconcave portion 25 is formed at the bottom face of the carrier 24, andan air permeable porous board 44 is contained in the concave portion 25The porous board 44 is connected to an air suction passage 46, and theair suction passage 46 is extended outside the wafer holding head 14 andconnected with a suction pump 48. Thus, as the suction pump 48 isactivated, the porous board 44 functions as a suction member, wherebythe porous board 44 holds a wafer 50 by suction. The porous board 44 hasa number of air passages therein, and a sintered body of a ceramicmaterial is used for example.

The outer periphery of the bottom face of the carrier 24 has air jettingopenings 52 formed concentrically with predetermined intervals. The airjetting openings 52 are provided through to ring-shaped air grooves 54formed at the bottom face of the carrier 24. The air jetting openings 52are also connected to an air supply passage 56 which is shown withalternative long and two short dotted lines in FIG. 2. The air supplypassage 56 is extended outside the wafer holding head 14, and isconnected with the air pump 40 via a regulator 58.

Therefore, as the air pump 40 is activated, compressed air from the airpump 40 is jetted downward from the air grooves 54 through the airsupply passage 56 and the air jetting openings 52. A pressurized airlayer 60 is therefore formed between the bottom face of the carrier 24and the reverse side of the wafer 50. The wafer 50 is pressed againstthe polishing pad 16 by the pressing force transmitted from the carrier24 through the pressurized air layer 60 and polished.

The disk-shaped rubber sheet 30 with a uniform thickness is fixed to thebottom face of the head body 22 by ring-shaped fixing metals 62, 64, and66 in large, medium, and small sizes by which the rubber sheet 30 isdivided in two, an outer part 30A and an inner part 30B. The inner part30B of the rubber sheet 30 divided as described above presses thecarrier 24, and the outer part 30A presses the retainer ring 28 via theguide ring 26. Additionally, the reference numbers 68 are bolts forconnecting the fixing metals 62, 64, and 66 with the head body 22.

Dividing the rubber sheet 30 in two as described above forms aring-shaped space 70 which is sealed by the inner part 30B of the rubbersheet 30 and a ring-shaped space 72 which is sealed by the outer part30A. Since the space 70 is connected to the air supply passage 34, theinner part 30B of the rubber sheet 30 is swollen by the air pressurewhen the compressed air from the air pump 40 is supplied from the airsupply passage 34 to the space 70. Consequently, the carrier 24, whichis positioned at the bottom of the inner part 30B, is pressed down bythe swell of the inner part 30B. Then the pressure force is transmittedto the wafer 50 through the pressurized air layer 60, so pressing forceof the wafer 50 against the polishing pad 16 can be obtained. Moreover,the pressing force of the wafer 50 against the polishing pad 16 can becontrolled by adjusting the air pressure by the regulator 38.

On the other hand, the outer part 30A of the rubber sheet 30 can beswollen by the air pressure as the compressed air from the air pump 40is supplied from the air supply passage 36 to the space 72 because theair supply passage 36 is connected to the space 72. Thus, the guide ring26, positioned at the bottom of the outer part 30A, is pressed down bythe outer part 30A, then the retainer ring 28, arranged at the bottom ofthe guide ring 26, is pressed against the polishing pad 16. Further, thepressing force of the retainer ring 28 can be controlled by adjustingthe air pressure by the regulator 42. The pressing force of the retainerring 28 is set to be the force in which stress from the polishing pad 16does not concentrate on the edge of the wafer 50.

The cylindrical guide ring 26 is arranged at the bottom of the head body22 to be coaxial with the central axis of the head body 22. A flange 26Ais also formed at the upper periphery of the guide ring 26 which, aselevating the wafer holding head 14 from the polishing pad 16, contactswith a flange 62A formed at the lower periphery of the fixing metal 62.Thus the guide ring 26 is prevented from falling off the wafer holdinghead 14.

The retainer ring 28 is a ring for preventing the wafer 50 from fallingoff the wafer holding head 14 during polishing, and is mounted to thebottom outer periphery of the carrier 24 via an O ring 74. As explainedabove, by mounting the retainer ring 28 to the carrier 24 via the O ring74, the O ring 74 absorbs the impact at contacting of the wafer 50 withthe retainer ring 28. The wafer 50 can be set to the carrier 24 easilydue to that the retainer ring 28 is held coaxially with the carrier 24.

Next, a polishing end detecting apparatus will be explained which isapplied to the wafer holding head 14 in the present embodiment.

The polishing end detecting apparatus in the present embodiment isconstructed considering the difference in the rotation torque of thecarrier 24 by the difference in processing quality of the wafer 50. Inother words, the polishing end detecting apparatus comprises a straingage 96 for determining the rotation torque of the carrier 24 as strain,and a control device 100 (same as the polishing amount determiningdevice) with a processor 98 for calculating the rotation torqueaccording to an electric signal outputted from the strain gage 96.

The strain gage 96 is attached to a connecting bar 80 which is a memberto suspend the carrier 24 to the head body 22 and is arranged to becoaxial with the central axis of the wafer holding head 14. Theconnecting bar 80 comprises a base 82, a connecting part 84, and aconstricted part 86. The base 82 is fixed to the head body 22 by a bolt(not shown), and the connecting part 84 is inserted into a rectangularconcave groove 88 which is formed on top of the carrier 24. As shown inFIG. 3, the connecting part 84 has a rectangular flange 90 formed at itsbottom, and the flange 90 is fit in the groove 88. Thus, the carrier 24is connected in a state to be prevented from rotating with respect tothe head body 22 while prevented from shaking. A ring 94 is fixed to thecarrier 24 with bolts 92 to prevent the carrier 24 from coming off overthe flange 90.

As shown in FIG. 2, the strain gage 96 is attached to the constrictedpart 86 of the connecting bar 80. The gage 96 measures the strain intwist (in a rotating direction) of the constricted part 86 as theelectric signal, which is outputted to the processor 98 of the controldevice 100. The processor 98 calculates the rotation torque of thecarrier 24 according to the electric signal outputted from the straingage 96. Then the control device 100 controls the wafer polishingapparatus 10 to terminate polishing the wafer 50 as the rotation torque,calculated by the processor 98, changes to the rotation torquecorresponding to the terminal material. The reference rotation torque isstored in a RAM (not shown) of the control device 100. The referencerotation torque is read out by the control device 100 at polishing, andis compared with an actual rotation torque.

As explained above, in order to determine the rotation torque of thecarrier 24 accurately, the head body 22 and the carrier 24 are connectedwith each other by the connecting bar 80 which is provided with thestrain gage 96, and the rotation torque of the carrier 24 is determinedaccording to the electric signal outputted from the strain gage 96.

Thus, as compared with a polishing end detecting device that determinesthe rotation torque of the carrier 24 according to an electricresistance of a motor rotating the wafer holding head 14, the polishingend detecting device in the present embodiment can accurately determinethe rotation torque, so that the polishing end of the wafer 50 can bedetected accurately.

In addition, instead of the strain gage 96, any other determining devicewhich can determine the rotation torque of the carrier 24, such as apiezoelectric sensor, may be used.

Further, in the present embodiment, a force F in the horizontaldirection of the carrier 24 is received on the central axis of the waferholding head 14; consequently, the freedom of the wafer holding head 14as to inclination can be widened more than in a case when the force F isreceived by the outer periphery of the wafer holding head 14.

Now, an operation will be explained of the wafer holding head 14 of thewafer polishing apparatus 10 constructed as described above.

First, the wafer 50 to be polished is held by the porous board 44 andthe wafer holding head 14 is moved to a predetermined position on thepolishing pad 16.

Second, the wafer 50 is released from the porous board 44, and the wafer50 is placed on the polishing pad 16.

Then, the air pump 40 is activated to jet the compressed air from theair supply passage 56 to between the carrier 24 and the wafer 50 throughthe air jetting openings 52 and the air grooves 54, and the pressurizedair layer 60 is formed between the carrier 24 and the wafer 50.

After that, the compressed air from the air pump 40 is supplied to thespace 70 through the air supply passage 34, and the inner part 30B ofthe rubber sheet 30 is swollen by the inner air pressure so as to pressthe carrier 24. By that process the wafer 50 is pressed against thepolishing pad 16 by the pressure force of the carrier 24 which istransmitted through the pressurized air layer 60. Then, the air pressureis adjusted by the regulator 38 to control the inner air pressure to adesired pressure, and the pressing force of the wafer 50 against thepolishing pad 16 is stabilized.

Following the above process, the compressed air from the air pump 44 issupplied to the space 72 through the air supply passage 36, and theretainer ring 28 is pressed via the guide ring 26 by swelling the outerpart 30A of the rubber sheet 30 by the inner air pressure, so that theretainer ring 28 is pressed against the polishing pad 16. Then, the airpressure is adjusted by the regulator 42 to control the inner airpressure to the desired pressure, and the pressing force of the retainerring 28 against the polishing pad 16 is maintained constant.

After that, the wafer holding head 14 and the platen 12 are rotated tostart polishing the wafer 50.

During the polishing, the outer periphery of the wafer 50 contacts withthe inner periphery of the retainer ring 28, so that the wafer 50 ispolished in a state that it is prevented from jumping out of the waferholding head 14. The impact at contact of the wafer 50 with the retainerring 28 is absorbed by the deformation of the retainer ring 28 and the Oring 74. Therefore, according to the wafer holding head 14, the wafer 50can be prevented from damage caused by the impact.

Moreover, the retainer ring 28 in the wafer holding head 14 is fixed tothe carrier 24 without a gap, thereby the wafer is polished in a statethat it is enclosed by the retainer ring 28. Therefore, according to thewafer holding head 14, the wafer 50 can be polished in a state that thecenter of the wafer 50 is held on the central axis of the carrier 24, sothat the polishing precision of the wafer 50 can be improved.

On the other hand, the polishing amount of the wafer 50 during polishingis always determined by determining the rotation torque of the carrier24 with the strain gage 96 and the control device 100. When thedetermined rotation torque is changed to be the rotation torquecorresponding to the terminal material, the control device 100 controlsthe wafer polishing apparatus 10 and terminates polishing the wafer 50.

After that, the polished wafer 50 is held by the porous board 44, andtransported to a cleaning apparatus of the next process by an unloadingaction of the wafer holding head 14. Then operation of the wafer holdinghead 14 is finished.

The present embodiment represents the explanation of an example in whichthe O ring 74 is applied as a buffer; however the invention is notlimited to that. Any embodiment is possible in which a member is appliedthat can buff the impact at contact of the wafer 50 with the retainerring 28.

FIG. 5 is a longitudinal section view of the wafer holding head 200 inthe second embodiment; the same or similar members as the wafer holdinghead 14 in the first embodiment are assigned the same numbers, and theexplanation for them is omitted.

The wafer holding head 200 has the head body 22 and the carrier 24connected with each other by parallel springs 202 between which adifferential transformer 204 is attached. As shown in FIGS. 5 and 6, acore 206 of the differential transformer 204 is attached horizontally toa spring 202A, one of the parallel springs 202, and a bobbin 208 of thedifferential transformer 204 is attached horizontally to a spring 202B,another of the parallel springs 202. The top end portion of the parallelsprings 202 is fixed to the fixing metal 66, and its bottom end portionis loosely fit and supported by a base plate 205 attached to the carrier24.

An electric signal outputted from the differential transformer 202 isoutputted to a control device 210 which is arranged outside the waferholding head 200 shown in FIG. 5. The control device 210 has anamplifier (not shown) for amplifying the electric signal, and aprocessor 212 for calculating the friction force occurring at the wafer50 and the polishing pad 16. The control device 210 also has a rectifiersmoothing circuit (not shown) for rectifying the amplified electricsignal.

As the friction force (friction coefficient) occurring at the wafer 50and the polishing pad 16 grows, a displacement amount of the carrier 24with respect to the head body 22 in the horizontal direction becomeslarge; thus, the displacement between the springs 202A and 202B of theparallel springs 202 becomes large (about 2-3 μm). Since thedisplacement is determined by the differential transformer 204, adetected signal (alternating current voltage signal) with a largeoscillation is outputted from the amplifier to the processor 212 asshown in FIG. 7. The processor 212 then calculates the friction forceaccording to the detected signal.

The processor 212 stores beforehand the friction force corresponding tothe oscillation of the detected signal, in which the friction forcecorresponding to the oscillation is read out when the detected signal isinputted, and the friction force is outputted to the control device 210as a calculated value. The control device 210 also stores beforehand thefriction force (oscillation A1 in FIG. 7) indicating that the polishingpad 16 needs dressing and the friction force (oscillation A2 in FIG. 7)indicating the polishing pad 16 needs to be replaced. The control device210 automatically determines the times for dressing and replacement ofthe polishing pad 16 according to the outputted friction force from theprocessor 212. Therefore, the wafer can be polished while maintaining apolishing rate constant.

In other words, the wafer holding head 200 in the second embodimentdetermines the force F of the carrier 24 in the horizontal direction asthe friction force of the wafer 50 against the polishing pad 16 due tochanges in quality of a membrane to be processed (the surfacecondition), and outputs an end detecting signal for stopping the processor progressing onto the next polishing as the friction force reaches theforce of the predetermined membrane quality. Moreover, an amount ofpolishing pad to be worn out under the dressing pressure and thedressing time is obtained beforehand, whereby a limit of using thepolishing pad 16 (its thickness about 1.5 mm) can be predicted.

The processor 212 can also calculate the rotation torque of the carrier24 applied from the carrier 24 to the parallel springs 202 according tothe voltage of the direct current which is rectified by the rectifiersmoothing circuit. The processor 212 stores beforehand the rotationtorque of the carrier 24 corresponding to the voltage of rectifieddirect current. The rotation torque corresponding to the voltage of thedirect current is read out, and outputted to the control device 210. Thecontrol device 210 stores beforehand the rotation torque (voltage V1 inFIG. 7) indicating the polishing end of the wafer 50. The control device210 automatically determines the end of the polishing amount of thewafer 50 according to the rotation torque outputted from the processor212.

On the other hand, in a case to dress the polishing pad 16, the dressingpressure and the time change are monitored until the polishing padreturns to its initial state (the initial friction coefficient=frictionforce), and if the time until the polishing pad is abraded to thethickness limit for use is predicted, exchanging of the polishing pad 16can be prepared beforehand.

Moreover, in the second embodiment, the parallel springs 202 are appliedas the connecting member, the differential transformer 204 fordetermining the displacement between the parallel springs 202 is appliedas the friction force determining device, and the processor 212calculates the friction force occurring at the wafer 50 and thepolishing pad 16 according to the electric signal outputted from thedifferential transformer 204; therefore, the friction force can beaccurately determined with a simple structure. In addition, anotherfriction determining device may be used instead of the differentialtransformer 204.

In the second embodiment, further, since the springs 202A and 202B ofthe parallel springs are symmetrically arranged with respect to thecentral axis of the head body 22 and the carrier 24, and the core 206 isarranged on the central axis, the friction force in the rotationdirection of the polishing pad 16 can be received by the central axis ofthe holding head 200. Therefore, the friction force can be determinedaccurately compared with determining the friction force at the peripheryof the holding head 202.

FIG. 8 is a longitudinal section view of a wafer holding head 300 in thethird embodiment. The same or similar members as the wafer holding head14 in the first embodiment are assigned the same reference numbers, andexplanations for them are omitted.

The wafer holding head 300 has the head body 22 and the carrier 24connected with each other by a pin 302 to which four differentialtransformers 304 are attached. A constricted part 303 is formed at thetop of the pin 302. As the force F in a horizontal direction is appliedto the pin 302, the pin 302 slightly deforms elastically in thehorizontal direction with the constricted part 303 as the fulcrum.

As shown in FIG. 9, four cores 306 of the differential transformers 304are attached to the periphery of the pin 302 with equal intervals in thehorizontal direction. Four bobbins 308 of the differential transformers304 are attached to an opening 310A with equal intervals in thehorizontal direction. The opening 310A is formed to a cylinder body 310forming the exterior of the pin 302. The top of the cylinder body 310 isfixed to the fixing metal 66, its bottom end part left free.

Electric signals, outputted from the four differential transformers 304are outputted respectively to a control device 312 arranged outside thewafer holding head 300 in FIG. 8. The control device 312 has anamplifier (not shown) for amplifying the electric signals, and aprocessor 314 for calculating the friction force occurring at the wafer50 and the polishing pad 16 according to the amplified electric signals.The control device 312 also has a rectifier smoothing circuit (notshown) for rectifying the amplified electric signals.

In the embodiment of the present invention, the four differentialtransformers 304 are provided, and alternating voltages whose phases areshifted by 90° are applied to the differential transformers 304. Thus anoscillation and a voltage can be detected at ¼ rotation of the waferholding head 300, thus a sampling time can be subdivided. In theembodiment, the four differential transformers 304 are provided. Thenumber of the differential transformers is not to be limited by four;one differential transformer may be used. However consideringsubdividing the sampling time, two or more would be preferred.

Furthermore, in the third embodiment, the pin 302 which can elasticallydeform is applied as the connecting member and the differentialtransformers 304 are applied as the friction force determining device,and the processor 314 calculates the friction force occurring at thewafer 50 and the polishing pad 16 according to the electric signalsoutputted from the differential transformers 304; therefore, thefriction force can be accurately determined with a simple structure.

As explained above, according to the present invention, the head bodyand the carrier of the holding head are connected with each other by theconnecting member to which the torque determining device or the frictionforce determining device is provided; thus the friction force can beaccurately determined. Therefore, a timing for dressing and a polishingrate can be predicted by the difference in the friction force of thewafer and the platen compared with the beginning of polishing can bepredicted, and thus the polishing end can be accurately detected.

Moreover, duration of the platen can be predicted by the change in thefriction force of the wafer and the platen after dressing.

Further, since the retainer ring is mounted to the outer periphery ofthe carrier via the buffer, the wafer can be prevented from damagescaused by the impact at contacting of the wafer to the retainer ring,and since the retainer ring is mounted to the carrier without a gap, theouter periphery of the wafer is enclosed by the retainer ring and thewafer can be polished in a state where its center is held on the centralaxis of the carrier. Therefore, the present invention can improve thepolishing precision of the wafer.

It should be understood, however, that there is no intention to limitthe invention to the specific forms disclosed, but on the contrary, theinvention is to cover all modifications, alternate constructions andequivalents falling within the spirit and scope of the invention asexpressed in the appended claims.

What is claimed is:
 1. A wafer polishing apparatus, comprising: a platenwhich rotates; and a holding head which holds a wafer and presses thewafer against the platen to polish an obverse of the wafer, the holdinghead comprising a head body which is rotated and arranged to face theplaten, a carrier which is supported to the head body to be movablevertically, an air blowing member which is provided at a bottom face ofthe carrier and blows air toward a reverse of the wafer to form apressurized air layer between the carrier and the wafer, a pressingdevice which presses the carrier toward the platen to press the waferagainst the platen through the pressurized air layer, and a retainerring which is mounted on an outer periphery of the carrier via a buffermember and pressed against the platen during polishing the wafer toprevent the wafer from jumping out of the carrier while enclosing thewafer to hold a center of the wafer on a central axis of the carrier.